1. Field of the Invention
The present invention relates to a sheet conveying apparatus used with an image forming apparatus or image reading apparatus such as a copying machine, a scanner, a printer and the like, and more particularly, it relates to a sheet conveying apparatus wherein registration of sheet is effected at an upstream side of an image forming portion or an image reading portion.
2. Related Background Art
Conventionally, in image forming or image reading apparatuses such as copying machines, scanners, printers and the like, a registration device for determining posture and position of a sheet is arranged immediately in front of an image forming portion or an image reading portion. As such registration devices, for example, there has been proposed a registration device of loop registration type wherein a tip end of a sheet is abutted against a nip between a pair of stopped rollers to form a loop in the sheet, so that the tip end of the sheet is aligned with the nip, thereby correcting skew-feed of the sheet, and a registration device of shutter registration type wherein a shutter member for stopping a tip end of a sheet is retractably arranged in a sheet conveying path. After the tip end of the sheet is abutted against the shutter member to correct skew-feed of the sheet, the shutter member is retracted from the sheet conveying path.
However, recently, as such image forming or image reading apparatuses have been changed into digital design, a distance between sheets (referred to as "sheet interval" hereinafter) has been shortened so that a large number of sheets can be treated within a short time period, thereby increasing substantial image forming speed without increasing a process speed for image formation.
For example, in conventional digital copying machines, when a copying operation is performed continuously, since an optical system for exposing an original must be reciprocally shifted by times corresponding to the number of copies, the sheet interval used in an image information copying operation is necessarily determined. However, when the original is digitally read and the image is digitally formed, after the original was read once, the image information is electrically coded to be stored in a memory. In the image formation, the image information is read out from the memory, and a latent image corresponding to the image information is formed on a photosensitive member disposed in the image forming portion by using an exposure device including a laser beam or an LED array. Thus, even when a plurality of copies are obtained, mechanical movement of the optical system and the like can be eliminated.
Accordingly, the time period for effecting the registration of the sheet is one of important factors for determining the sheet interval. In order to reduce the registration time period, there has been proposed an active registration method for correcting the skew-feed of the sheet while conveying the sheet. In this method, two sensors are arranged in a sheet conveying path and disposed in a plane perpendicular to the sheet conveying path, so that a tip end of the sheet moving in the sheet conveying path is detected by the sensors, respectively, to determine inclination of the sheet on the basis of signals from the sensors, and the skew-feed of the sheet is corrected by controlling sheet convey speeds of skew-feed correction rollers (referred to as "regist rollers" hereinafter) arranged in a plane perpendicular to the sheet conveying path and drive independently. According to this method, since the skew-feed of the sheet can be corrected while conveying the sheet without stopping the sheet temporarily, the sheet interval can be reduced in comparison with other methods.
However, in the above-mentioned conventional example, if a size of the sheet to be conveyed is not constant, and particularly, if a long-sized sheet is conveyed, the correction of the skew-feed of the sheet must be effected by the regist rollers under a condition that a trailing end of the sheet is pinched between convey rollers disposed at an upstream side of the regist rollers in the sheet conveying direction. That is to say, in the above-mentioned active registration method, although a slower side (succeeding side) of the sheet is advanced or a faster side (preceding side) of the sheet is delayed by means of the regist rollers, in both cases, since the entire sheet must be rotated, if the trailing end of the sheet is pinched between the convey rollers, it will be hard to rotate the sheet by a desired amount, with the result that it is very difficult to correct the skew-feed of the sheet with high accuracy.
In order to eliminate the above drawback, there has been proposed a method wherein a loop is formed in the sheet in front of the regist rollers and the rotational movement of the sheet generated in the skew-feed correction is absorbed by distorting the loop. However, in this method, since the loop is released as soon as the trailing end of the sheet leaves the convey rollers, if the sheet interval is small, there is a danger of the collision of the trailing end of the sheet against a tip end of a next sheet. Thus, a certain amount of sheet interval or more must be maintained.